1. Field of the Invention
The present invention relates to a process for preparing tris(silyl)methanes by directly reacting a mixture of .alpha.-dichloromethylsilanes represented by formula I and hydrogen chloride or alkyl chlorides represented by formula II, with silicon metal to give the tris(silyl)methanes having two dichlorosilyl groups (formula III) and the tris(silyl)methanes having one trichlorosilyl group and one dichlorosilyl group (formula IV) in moderately high yields in the presence of copper catalyst at a temperature from 250.degree. C. to 350.degree. C. A different major product is obtained depending upon the alkyl chloride incorporated. For example, n-butyl chloride, t-butyl chloride, and propyl chloride gave tris(silyl)methanes with one hydrogen substituted on each of the two silicon atoms as the major product. When 1,2-dichloroethane is incorporated, tris(silyl)methanes having two trichlorosilyl group is the only major product. The preferred reaction temperature range is 300.degree.-330.degree. C. Useful copper catalysts include copper metal, copper salts, partially oxidized copper. ##STR2## In the formulas, R represents hydrogen, alkyl(C.sub.1 -C.sub.4), or CH.sub.2 CH.sub.2 Cl, and R.sub.1, R.sub.2 and R.sub.3 represent independently hydrogen or chloride.
2. Description of the Prior Art
Methylchlorosilanes are the most important starting materials for silicons. E. G. Rochow discovered the direct process for the synthesis of methylchlorosilanes, reacting elemental silicon with methylchloride in the presence of a catalyst in 1940 (E. G. Rochow U.S. Pat. No. 2,380,995). EQU Si+2CH.sub.3 Cl.fwdarw.(CH.sub.3).sub.2 SiCl.sub.2
The reaction gives dimethyldichlorosilane, methyltrichlorosilane, trimethylchlorosilane, and tetrachlorosilane. A number of high boiling compounds are also found in the mixture of the products in a small quantity. The reaction rate and the nature of products depend on a large number of factors. These determining factors include the nature of starting materials, catalyst, reaction temperature, reaction pressure, the type of reactor used, and the degree of conversion of silicon and methyl chloride.
The catalyst for the direct process is always copper, in some cases co-catalysts such as zinc, aluminum, cadmium, etc. are added. The co-catalysts enhance the reactivity of silicon metal and shorten the induction period and increase the selectivity of dimethyldichlorosilane production. The reaction is carried out at 250.degree.-350.degree. C., and the yield of dimethyldichlorosilane decrease at temperatures above 300.degree. C. In the absence of a catalyst, the reaction is sluggish and gives irreproducible results (E. G. Rochow, J. Am. Chem. Soc., 67, 963 (1945)). The composition of products depends on the amount of copper used. The greater amount of copper is used, the higher is the chlorine content of the resulting products. The greatest catalytic efficiency is obtained when the amount of copper is 10% of the amounts of silicon.
The reactivity of the silicon-copper mixture is connected with the formation of an intermetallic .eta.-phase (Cu.sub.3 Si). The presence of the .eta.-phase in the mixture is of fundamental importance for the seletive synthesis of dimethyldichlorosilane. It is known that the mixture of silicon powder and copper powder is heated 800.degree. C. to 1000.degree. C. in nitrogen, or better in hydrogen, the powders become sintered and the .eta.-phase in formed (P. Trambouze and B. Imelik, J. Chem. Phys. 51, 505 (1954)). The .eta.-phase is also chemically prepared by heating cuprous chloride with silicon at a temperature above 350.degree. C. (E. G. Rochow in Inorganic synthesis, III, McGraw-Hill, New York 1950, p56) EQU nSi+CuCl.fwdarw.SiCl.sub.4 +Cu.sub.3 Si+Cu+(n-2)Si
The reaction rate and the composition of the products in the direct process are highly temperature-dependent (A. L. Klebamskii and V. S. Fikhtengolts, J. Gen. Chem. U.S.S.R., 27, 2693 (1957)). It is very important to maintain the reaction temperature at an accurately specified temperature and to prevent and hot spot developing in the agglomerates of the solid phase. It is reported that at higher temperatures, the deposition of carbon on the surface of the metal mixture occur which slows down the reaction (J. C. Vlugter, and R. J. H. Voorhoeve, Conf. Accad. Lin-cei, Alta Tech. Chim. 1961 p81(1962)). This is why the reactor for the direct synthesis of methylchlorosilane must have a high thermal stability and an efficient heat transfer.
The direct process can be carried out in fixed bed, in strired bed, and also in fluidized bed reactors. The process with the stirred bed reactors has the advantages over the fixed bed operation that the heat of reaction can be removed more easily and the movement of the powders causes fresh surface to be continuously exposed. Sellers and Davis reported that a mechanically stirred fluidized bed could be used (J. E. Sellers and J. L. Davis, U.S. Pat. No. 2,449,821). The metal powder was agitated in an up and down motion in a vertical reactor by means of spiral band rotated by a central shaft while a stream of methyl chloride was moved up-ward through it. Bluestrim used a fluidized bed reactor for the production of methylchlorosilane (B. A. Bluestrim, U.S. Pat. No. 2,887,502).
Petrov et al reported the preparation of chlorosilaalkanes by reacting silicon metal with .alpha.-dichloromethylsilanes. The reaction of .alpha.-dichloromethylsilanes with silicon metal at 360.degree. C. gave 14% yield of tris(trichlorosilyl)methane and about 70% by-products due to the decomposition of the starting material (A. D. Petrov, S. I. Sahykh-Zade, E. A. Chernyshev. V. F. Mironov, Zh. Obschch. Khim., 26 1248 (1956)). The expected tetrakis(silyl)methane was not obtained when bis (trichlorosilyl)dichloromethane was reacted with metallic silicon. All the products obtained were from the secondary reaction between metallic silicon and the compounds produced from the decomposition of bis(trichlorosilyl) dichloromethane. Several year later, Muller and his co-workers also studied the same reaction and reported that tetrakis (silyl)methane was not produced but the starting material decomposed (R. Muller and H. Beyer, Chem. Ber., 92, 1957 (1959); 96, 2894 (1963)).
We reported that trisilalkanes as the major products and bis(silyl)methanes as the minor products were prepared by reacting .alpha.-chloromethylsilanes with metallic silicon in the presence of copper catalyst at a temperature from 250.degree. C. to 350.degree. C. The copper catalyst was used in an amount of the 1-20% of total contact mixture, but the preferred amount was 5-10%. The reaction could be carried out in a fluidized bed or in a stirred bed reactor. Addition of microspherical acid clay to silicon metal improved the fluidization and gave better results (I. N. Jung, G. H. Lee, S. H. Yeon, M. Suk, U.S. Pat. No. 5,075,477 (1991. 12. 24)).
The reaction can be illustrated as follows: ##STR3## wherein R.sub.1, R.sub.2, R.sub.3 may independantly be chloride or methyl.
We also reported the direct synthesis of Si--H containing bis(silyl)methanes by reacting silicon metal with a mixture of .alpha.-chloromethylsilanes and hydrogen chloride. The bis(silyl)methane containing dichlorosilyl group was obtained as the major product and bis(silyl)methane containing trichlorosilyl group was obtained as the minor product. The major product could be explained by the reaction of the same silicon atom with each mole of two starting materials. The results suggest that the reactivities of the two starting materials were not much different. The major portion of the other by-products was trichlorosilane and tetrachlorosilane which were produced from the reaction between silicon metal and hydrogen chloride. The same results were obtained when hydrogen chloride was substituted by alkyl chlorides such as 1,2-dichloroethane, propyl chloride, n-butyl chloride or t-butyl chloride, because alkyl chlorides decomposed to give off hydrogen chloride (Korean Patent Application No. 91-24243). ##STR4## wherein R.sub.1, R.sub.2, R.sub.3 may independently be chloride or methyl.
We also found that bis(silyl)methanes having two dichlorosilyl groups at the both ends of the molecule along with bis(silyl)methane having two trichlorosilyl groups at the both ends and bis(silyl)methane having one dichlosilyl group and one trichlorosilyl group at each end were obtained by reacting a mixture of methylene chloride and hydrogen chloride (Korean Patent Application No. 92-935). ##STR5##